Sizing compositions can be used to improve certain processing properties of glass or carbon fibers, such as, for example; fiber bundle cohesion, bundling, spreadability, fuzz formation resistance, fiber smoothness and softness, abrasion resistance and easy and nondestructive unwindability of bobbined fiber bundles. Sizing compositions can also affect the physical properties of the composite containing the treated fibers.
The reinforced plastics industry has used glass fibers in various forms for reinforcing polymeric matrices to produce a variety of products. Glass fibers have been used in the form of continuous and chopped filaments and strands and rovings and woven and nonwoven fabrics to reinforce polymers. Thermosetting polymeric matrices have been reinforced with a variety of different forms of glass fibers resulting in the production of products such as: sheet molding compounds, bulk molding compounds, pultrusion products, panel products, spray up molding products, etc.
Production of glass fibers for the polymeric reinforcement market involves, attenuation of the glass fibers from molten streams of fiberizable glass material from a bushing or like device connected to a furnace containing molten fiberizable glass material. The glass fibers are attenuated by conventional means such as winders or pull wheels. In the process of producing glass fibers, a chemical composition is applied to them shortly after they are attenuated as the molten streams of glass. Prior to the present invention, the chemical composition has traditionally been an aqueous solution, foam or gel composition containing film forming polymeric materials, coupling or keying agents, lubricants and sometimes processing aids. This chemical composition or sizing is necessary in order to retard inter filament abrasion of the glass fibers when they are gathered into a bundle of glass fibers or strands. It is also required in order to make the glass fibers compatible with polymer matrices that they are used to reinforce. After application of the sizing, the fibers are then dried either in the package form or in the chopped strand form before they are used for reinforcement.
In addition to glass fibers, carbon fibers have been found to combine excellent mechanical properties, such as high tensile strength and high modulus of elasticity, on the one hand, and low-density, high heat resistance and chemical resistance on the other. This combination of properties has led to the increased use of these materials as reinforcing elements in composite materials for a wide range of applications in industries as diverse as aerospace, transportation and sporting goods.
Optimal properties of carbon fibers are only obtained if integral adhesion between the matrix material and the reinforcing fiber is ensured over a wide range of varying temperature and moisture conditions. To achieve this, carbon fibers are given an oxidative surface treatment and subsequently provided with a sizing agent suitable for that fiber and its intended use. Sizing is used to prevent the formation of broken filaments (i.e., fuzz) during processing and weaving of the carbon fiber tow bundle.
Not only is it necessary that the sizing protect the fiber and impart improved physical properties, but it must also be chemically compatible with the particular matrix material so as to produce the qualitatively high-grade and durable composite materials. Sizing agents can be grouped into two types: solvent and emulsion types. In emulsion types, the resins are dispersed in water with the aid of dispersants and/or emulsifiers. In the solvent type, the polymer, usually a resin, is in solution in a low-boiling organic solvent and is applied to the fibers from a dilute solution. Both forms of sizing can add VOCs to the production environment. This adds time and expense to the production process.
Previous aqueous sizings have contained a significant amount of volatile organic components. The industry, in an effort to prevent environmental problems, has been attempting to find ways of minimizing levels of VOCs while maintaining the physical properties of the fibers.
Thus, there is a need in the art for a sizing composition which contains little or no volatile organic compounds and is capable of providing desired physical properties to treated fibers.